We'd like to convert from a two-handled wall-mounted (with a wall-mounted diverter) bathtub faucet to a single-handled faucet with the diverter on the spigot. Is this something that a relative novice can do, or do we need to call on a pro?

If you have never tackled a project like this before, I'd recommend calling in a plumber.

If the diverter is wall mounted, does that mean you actually have a three handled faucet? Or is the diverter separate from the hot and cold controls?

I suppose it's a 3-handled faucet, then. The diverter knob is between the hot and the cold knobs.

Right. That makes it a little harder, since there are "single handle" faucets that use two holes, but not three. The temperature control goes in one hole after the hole is made larger, and the volume control fits the other side. With a three handle faucet, you need a chrome cover plate if you do not want to do any retiling. The cover plates come in various configurations so you can use the standard single handle faucet assuming you can get to the piping to make the revision.

Other than running some sweated copper to bypass the water softener to the kitchen cold faucet,

this project... of replacing an old 3 knob faucet set with a single moen posi-temp was my first project of its kind.

It required some repiping, which required some sweating again, but my tile was laid onto concrete on steel mesh, so nothing caught fire.

It helps to ask for advice.

moen makes a plate to change a 3 handle fauset to 1
go to moen.com and look for a remodel plate

"Smitty" also makes remodel plates in the different configurations.